|The landscape, wandering with a mind of its own, travels today at the speed at which fingernails grow. Los Angeles creeps toward San Francisco; the Palos Verdes Peninsula squeezes toward Pasadena; and the Ventura basin is slipping shut by a centimeter or so every year, geodetic satellite studies show. California's mobility is hardly unique. Texas was part of Argentina 535 million years ago, new studies suggest. Africa, India, Australia and Antarctica were of a single piece in the distant past, geophysical research indicates. More recently, India rammed Asia at a breakneck speed of 19 centimeters a year and lifted up the Himalayas.
This is the stuff of plate tectonics: Vast slabs of crust ride on upwellings of molten rock from deep within the mantle of the Earth, like slips of paper buoyed by the circulation of air--rising with hot updrafts and sinking on the cooler downward flows. Pushing parts of the Earth's crust together or pulling them apart, these subterranean convection currents are an engine of disruption that builds mountains, opens oceans and elbows the continents--one earthquaking jolt at a time--into new positions.
When first proposed a generation ago, the theory of continental drift and plate tectonics was one of the most shocking scientific ideas of its day. But now a team of Caltech geophysicists suggests that virtually all of life today owes its existence to an even more radical form of Earth motion. By their theory, it was not the continents that moved, however, but the entire outer layer of the planet. In plate tectonics, fragments of crust move independently along the surface of the viscous mantle. But in this hypothesis, the continents, oceans and the mantle itself all shifted in unison around the planet's liquid center--and at an extremely rapid rate.
Caltech geophysicists Joseph Kirschvink and David Evans said this unprecedented revolution of the Earth's crust about 530 million years ago triggered an extraordinary episode in the evolution of life called the Cambrian explosion. In this event, about 95% of the types of life forms present today suddenly appeared in the fossil record, including the primitive vertebrate ancestors of humanity. Life, they say, was responding to a kind of planetary somersault--called true polar wander--in which the whole surface of the Earth was thrown off balance by the weight of an ancient supercontinent.
The entire crust slid in one piece around the planet's molten core, moving from the pole to the equator about twice as quickly as the fastest known tectonic plate movement. The rolling of the crust churned the oceans, disrupted climate patterns and accelerated the pace of evolution to a fever pitch never again to be duplicated in the planet's history, they say in research published recently in Science.
The evidence of their hypothesis comes from ancient rocks that record Earth's magnetic field, which allows researchers to reconstruct how the position of ancient continents changed relative to the magnetic North Pole. "We wanted some unifying theory that could account for the explosive burst of evolution and we had all these rapid motions of the continents," Kirschvink said. "The continents basically are going in the same direction at breakneck speed. That does not sound like normal plate tectonics. "The simplest argument is that it is not plate tectonics. It is something else," he said.
Several experts in plate tectonics and geodynamics, however, were intrigued by but skeptical of the hypothesis. "I guess I would say: Oh really?" mused Elridge Moores, an authority at UC Davis on plate tectonics. "It does have an element of plausibility and it really does need to be seriously considered, [but] they have not convinced me." Like a number of geologists, he was not certain anything more elaborate than plate tectonics was needed to explain the movement of the continents during the Cambrian period. But the Caltech group said their research showed that the crust moved together at an absolute speed of about 30 centimeters a year--far in excess of any plate motion observed today.
"One rather important point is that there has been a plate tectonic speed limit and, with the true polar wander event, they are talking about the apparent velocity [of crust movement] that may be considerably greater," said Bill Thomas, a plate tectonics expert at the University of Kentucky.
But there is so little evidence available from the period under investigation that no one really knows how continental plates may have behaved so long ago. Some researchers have theorized that the tectonic motion may have been much faster earlier in Earth's history when the interior of the planet may have been warmer and its mantle more fluid. "Is this really some special event or is it a variation on a theme?" Moores said. "We don't know how much the average rate of motion might have increased as you go back in time."
Mark Richards, an expert at UC Berkeley on geodynamics, said he was "excited" by the hypothesis. Richards has developed sophisticated computer models of the spinning Earth and the behavior of tectonic plates. His simulations clearly show that this sort of polar wander event may not be as unusual as it first appears. A polar wander event could occur every 500 million years or so, his computer studies indicate. "There is nothing special about the dynamics required," he said. "What we know about the fluid properties of the mantle is consistent with this hypothesis. It is a startling idea, but just because it is startling does not mean it is nutty."
The more gradual motion of tectonic plates today may guard against such spectacular rearrangements of the Earth's surface by ensuring that weight always is safely distributed around the globe, Kirschvink said. Certainly, the timing of the polar wander event does coincide with a dramatic increase in the diversity of life forms, as preserved in fossils.
Most biologists recognize the remarkable conditions of the Cambrian period, but they aren't certain whether Earth movement had any effect on the pace of evolution. Some believe that the biological flowering of diversity in the Cambrian started well before the onset of the polar wander event proposed by Kirschvink and Evans at Caltech. "There really were some weird things geochemically and geophysically," said evolutionary biologist Gregory Wray at the State University of New York in Stony Brook. "The difficulty and the frustration always is to link an event that clearly happened in the rock record to an evolutionary event. The cause and effect is awfully hard to establish clearly," he said. Moreover, recent molecular biology research has some biologists no longer sure whether there was even anything out of the ordinary about evolution in the Cambrian.
Wray's own research into molecular evolution suggests that the diversity of the animal kingdom arose on Earth at least a billion years ago, twice as long ago as the fossils suggest. That means the pace of natural selection during the Cambrian may have been far more gradual than previously believed. In other words, the Caltech theory may be an answer in search of a question.